The present invention relates to an aluminum nitride sintered body with a high thermal conductivity for use in an insulating substrate, heat sink and the like. More particularly, it relates to a high thermal conductive aluminum nitride sintered body which is easily metallized.
In recent years, the requirements for minimizing elecronic instruments and improving their functions have increased prominently. As a result, semiconductors are rapidly developing towards improvements in Integration density, multifunctionization, high speed, high output and high responsibility, wherein more and more heat is generated by the semiconductor. Therefore, a base plate with higher heat-dissipation (or radiation) ability is needed instead of conventional Al.sub.2 O.sub.3 base substrates.
As the materials for substrates with high heat-dissipation ability, i.e., as the materials with high thermal conductivity, there are included, for example, diamond, CBN (cubic boron nitride), SiC (silicon carbide), BeO (beryllia), AlN (aluminum nitride), Si, and the like. However, these materials have following drawbacks. Diamond and CBN have a difficulty in manufacturing the product of such size usable as a substrate and are very expensive. SiC cannot be used instead of an Al.sub.2 O.sub.3 substrate since SiC is semiconductive and inferior in electric characteristics such as electric insulating ability, dielectric constant and the like, compared with Al.sub.2 O.sub.3. BeO, in spite of its excellent electric characteristics, has a difficulty in its stable supply since it is not manufactured in Japan depending on the import from foreign contries, due to the poisonous powder produced in the compacting process, abrasive machining process and the like.
Si has inferior electric characteristics and a low mechanical strength, and hence has a limited use as the material for the substrate. Although AlN has advantages that it is excellent in electric characteristics, for example, high insulating ability, high dielectric brake down strength, low dielectric constant, etc. and that it can be sintered under atmospheric pressure, it has a drawback that the metal layer cannot be formed on the required surface. In AlN, therefore, a substrate with multi-layers has never been developed. That is, AlN cannot be easily metallized because of a low wettability to metal, and hence it has a difficulty in use as a substrate.
For example, Japanese Patent Kokai Publication Nos. 50-75208 and 59-40404 disclose oxidizing of the surface of an AlN base plate before metallization, and Japanese Patent Kokai Publication No. 53-102310 discloses formation of a metal oxide layer on the surface of AlN base plate before metallization.
The above-disclosed techniques, in spite of the possibility of the metallization on the AlN sintered product, have the drawbacks that they cannot be employed in a simultaneous sintering for multilayer-products like laminated circuit boards or substrates thereof, and that they always deteriorate the thermal conductivity caused by formation of a low thermal conductive layer (e.g., oxide layer) between a metallized metal layer and the AlN substrate.